Seismic visibility of a deep subduction channel – insights from numerical simulation of high-frequency seismic waves emitted from intermediate depth earthquakes

Abstract

Abstract. Return flow in a deep subduction channel (DSC) has been proposed to explain rapid exhumation of high pressure-low temperature metamorphic rocks, entirely based on the fossil rock record. Supported by thermo-mechanical models, the DSC is envisioned as a thin layer on top of the subducted plate reaching down to minimum depths of about 150 km. We perform numerical simulations of highfrequency seismic wave propagation (1-5 Hz) to explore potential seismological evidence for the in situ existence of a DSC. Motivated by field observations, for modeling purposes we assume a simple block-in-matrix (BIM) structure with eclogitic blocks floating in a serpentinite matrix. Homogenization calculations for BIM structures demonstrate that effective seismic velocities in such composites are lower than in the surrounding oceanic crust and mantle, with nearly constant values along the entire length of the DSC. Synthetic seismograms for receivers at the surface computed for intermediate depth earthquakes in the subducted oceanic crust for models with and without DSC turn out to be markedly influenced by its presence or absence.While for both models P and S waveforms are dominated by delayed high-amplitude guided waves, models with DSC exhibit a very different pattern of seismic arrivals compared to models without DSC. The main reason for the difference is the greater length and width of the low-velocity channel when a DSC is present. Seismic velocity heterogeneity within the DSC or oceanic crust is of minor importance. The characteristic patterns allow for definition of typical signatures by which models with and without DSC may be discriminated.The signatures stably recur in slightly modified form for earthquakes at different depths inside subducted oceanic crust. Available seismological data from intermediate depth earthquakes recorded in the forearc of the Hellenic subduction zone exhibit similar multi-arrival waveforms as observed in the synthetic seismograms for models with DSC. According to our results, observation of intermediate depth earthquakes along a profile across the forearc may allow to test the hypothesis of a DSC and to identify situations where such processes could be active today.

Textures of eclogites and blueschists from Syros island, Greece: Inferences for elastic anisotropy of subducted oceanic crust

“…In reflection seismology, smaller scales, like the tectonic setting of a subduction channel, are considered. Recent numerical simulations of seismic wave propagation modeled a detailed subduction channel structure with mafic blocks in a serpentinite matrix [ Friederich et al ., ].…”

B, Sr and Pb isotope geochemistry of high-pressure Alpine metaperidotites monitors fluid-mediated element recycling during serpentinite dehydration in subduction mélange (Cima di Gagnone, Swiss Central Alps)

“…Bebout and Barton, 2002;Bebout, 2007;Marschall and Schumacker, 2012;Spandler and Pirard, 2013;Angiboust et al, 2014). Geophysical data and modelling studies suggest persistence of such layers atop of subducting plates down to sub arc depths (Bostock et al, 2002;Gerya et al, 2002;Syracuse et al, 2010;Friederich et al, 2014).…”

“…6; Syracuse et al, 2010) and geophysical imaging of subduction zones (e.g. Bostock et al, 2002;Friederich et al, 2014) encourages the use of these rocks to understand the role of deserpentinization of Gagnone-type peridotites in affecting the isotopic signature of arc lavas (Figs. 9 and 10).…”

Forced Return Flow Deep in the Subduction Channel, Syros, Greece

“…Penniston‐Dorland et al () argued that at, depending on bulk composition and pressure, temperatures of 500–700 °C amphibole and pyroxene form at the expense of the sheet silicates producing drier rocks. Because of this progressive dehydration at increasing depth less fluids are available for the hydration of the mantle wedge and it is likely that the subduction channel becomes narrower downward as illustrated schematically by Gerya et al (, their Figure 3), Agard et al (, their Figure 1), Vannucchi et al (, their Figure 1), Friederich et al (, their Figure 1), and Penniston‐Dorland et al (, their Figure 1). In other words, dehydration reactions and mantle hydration control, in part, the shape of a downward tapering channel (Figure 1).…”